The present invention relates to an electromagnetic ultrasonic transducer for impinging ultrasonic waves upon a work piece which is made of an electrically-conductive material or for receiving ultrasonic waves. The transducer comprises a coil support, which is made of a material having good magnetic-conductivity and which has a comb-like structure having parallel running channels and fins as well as HF coils, which are electrically interconnected in series and which are wound onto at least some of the fins at a distance from the radiation side of the transducer formed by the faces of the fins and in such a manner that the adjacent coils have an alternating direction of wind. Furthermore, the ultrasonic transducer has means for generating a static magnetic field.
In using such a transducer, for example, for nondestructive testing of electrically conductive or ferromagnetic work pieces for internal and surface faults, the transducer is placed with its radiation side in the vicinity of the surface of the to-be-tested work piece. Applying a high-frequency current to the HF coils of the transducer induces eddy currents in the work piece. These eddy current lead, under the influence of the magnetic field, to propagation of the ultrasonic waves in the work piece. In the same manner, such a transducer can receive ultrasonic waves reflected at faults in the work piece and convert them into high-frequency current pulses. This can occur using the same HF coils or using a separate set of coils. Due to the noncontact generation of ultrasonic waves in the work piece, such transducers offer particular advantages in testing work pieces in motion.
In the prior art embodiments of electromagnetic ultrasonic transducers, there is a difference between transducers that generate polarized transverse waves, so-called SV waves (shear vertical waves), in the plane of incidence and transducers that generate horizontally polarized transverse waves, so-called shear horizontal waves (SH waves), in the work piece or receive such shear horizontal waves from the work piece. The type of generation differs in both cases essentially in the kind and direction of the magnetic field which acts on the eddy current induced by the HF coils.
For instance, DE 36 37 366 A1 discloses an electromagnetic ultrasonic transducer for generating vertically polarized transverse waves (SV waves) provided with a coil support having a comb-like structure with parallel running channels and fins on which a plurality of HF coils, which are interconnected in series, are wound in alternating wind direction. By maintaining a minimum distance between the coil windings and the fin faces forming the radiation side of the transducer, the HF coils are protected from damage due to coming into contact with the to-be-tested work piece. In order to generate the required magnetic field, a permanent magnet or an electromagnet is disposed over the comb-like coil support. This magnet supplies a homogeneous static magnetic field in the region of the surface of the work piece.
The transducer periodicity of such an electro-magnetic ultrasonic transducer is determined by the distance between the single fins of the comb structure and the type of winding of the HF coil. Half the transducer periodicity corresponds to the distance between adjacent fins. Sound radiation occurs perpendicular to the fins of the coil support.
DE 36 37 366 A1 also discloses a preferred embodiment of an ultrasonic transducer in which the generated magnetic field is directed parallel to the longitudinal side of the fin and parallel to the surface of the work piece in order to generate transverse waves (SH waves) polarized perpendicular to the plane of incidence. In such a case, generating the ultrasonic waves in the work piece occurs via a purely magnetostriction mechanism. In this case, the magnetic field is aligned parallel to the eddy currents induced in the work piece. The disadvantage of this method of generation is, however, that generation effectiveness and reception amplitude are strongly dependent on the mechanical and metallurgical properties of the surface of the work piece. Furthermore, this type of generation requires that the work piece be made of a ferromagnetic material.
In order to avoid these problems, transducers with a periodic permanent magnet arrangement, in which the magnetic field is essentially directed perpendicular to the surface of the work piece, are used for ultrasonic generation of horizontally polarized transverse waves (SH waves). Prior art transducers comprise two rows of permanent magnets, on the radiation side of which a rectangular coil is wound. Such an assembly, as is shown by way of example schematically in FIG. 1, generates particle excursions in electrically conductive work piece materials when HF currents are applied to the coils due to the Lorentz force acting on the eddy currents induced in the surface of the work piece as a result of the magnetic field. These particle excursions lead to the generation of SH waves.
A further improvement of this type of transducer for generating polarized transverse waves perpendicular to the plane of incidence is disclosed in EP 0 609 754 A2. In this embodiment, at least four rows of alternating permanent magnet segments are provided, with two adjacent rows staggered a quarter of the periodicity of the single permanent magnets of each row respectively along their longitudinal axis. Each adjacent permanent magnet arrangement is provided with its own high-frequency coil. These high-frequency coils can be impinged with a HF signal shifted 90xc2x0 between the two high-frequency coils. In this manner, it is achieved that this ultrasonic transducer, both as transmitter transducer and receiver transducer, is provided with a one-sided direction characteristic with a single main radiation direction so that an improved signal/noise ratio is yielded in the nondestructive testing of the work piece.
However, when using such electromagnetic ultrasonic transducers, the sensitive coil wires lie between the permanent magnets and the surface of the work piece. As the coils must be placed very closely to the surface of the work piece in testing of the work piece in order to generate the ultrasonic amplitudes required for sufficient signal/noise ratio, there is greater risk of damaging and wearing the HF coils. Effective protection against wearing these coils is very difficult to realize in these transducers, because a protective covering would lead to undesired dampening of the amplitudes.
DE 42 23 470 discloses another electromagnetic ultrasonic transducer for generating high-frequency SH waves. This transducer comprises a toroidal strip core on which the windings of the HF coil are disposed at a distance from the faces, i.e. at a distance from the surface of the work piece, which reduces the risk of damaging or wearing the windings when using the transducer. This principle using toroidal strip cores is, however, not suited for setting up low-frequency SH wave transducers with large aperture widths, because only little generation effectiveness can be achieved.
Based on this state of the art, the object of the present invention is to provide an electromagnetic ultrasonic transducer for generating horizontally polarized transverse waves (SH waves), which are in particular also suited for the low-frequency SH wave range and permits testing work pieces made of electrically conducting materials with little risk of wearing for the HF coils.
The object of the present invention is solved using the electromagnetic ultrasonic transducer according to the claims. Advantageous embodiments of the electromagnetic ultrasonic transducer are the subject matter of the subclaims.
The present ultrasonic transducer comprises a coil support made of a material having good magnetic conductivity, i.e. a highly permeable material, which is provided with a comb-like structure having at least almost parallel running channels and fins as well as a plurality of HF coils which are electrically interconnected in series and are wound on at least some of the fins at a distance from a radiation side, which is formed by the faces of the fins, of the transducer in such a manner that the adjacent coils have an alternating wind direction. The radiation side corresponds in this transducer, as in the prior art transducers of the cited state of the art, to the bottom side of the transducer facing the work piece during testing. Furthermore, the transducer comprises a plurality of rows of permanent magnets which are disposed in the channels of the transducer in the longitudinal direction of the fins and have an alternating polar assignment.
The present electromagnetic ultrasonic transducer, referred to in the following also as electromagnetic SH wave transducer, is particularly suited for ultrasonic testing electrically conductive or ferromagnetic materials. Due to the special design of the core material, i.e. the coil support and the corresponding arrangement of the permanent magnets, the highly sensitive wires of the HF coils are raised a few millimeters from the surface of the to-be-tested work piece. The distance between the coil windings and the face of the fins, which form the bottom side respectively the radiation side of the transducers, is preferably about 6 to 8 mm. In this manner, the disadvantage of having to place the sensitive HF coils very proximate to the surface of the work piece in order to effectively protect the transducer against wear is avoided. The present transducer can be used particularly for generating low-frequency SH waves.
Contrary to the transducer periodicity of the transducer of DE 36 37 366 A1, the transducer periodicity is not determined by the distance between two adjacent fins of the coil support but rather by the periodicity of the arrangement of the magnets, i.e. by the (center of gravity) distance respectively the length of the single permanent magnets of a row of permanent magnets, with the length referring to the dimension in the longitudinal direction of the fin. The wave length projected on the surface of the workpiece and thus also the periodicity of the transducer is therefore oriented parallel to the channels respectively the fins of the coil support.
In the present transducer, the HF coils themselves are wound about the individual fins, with the wind direction of the HF coils of adjacent fins alternating. The electrical connection of these coils yields a coil arrangement course, which the cited state of the art also refers to as meandering, over the entire radiation surface of the transducer.
Due to the HF coils, which are electrically interconnected in series, of the single fins, dynamic magnet fields are impressed in the material of the to-be-tested work piece. These dynamic magnetic fields generate eddy currents therein. In the present transducers, these eddy currents run essentially under the permanent magnets, which have alternating pole assignment, in the direction of the channels of the comb. The permanent magnets generate a spatially periodic magnetic field essentially perpendicular to the surface of the work piece. In the adjacent channels, the current direction runs in the opposite direction. Due to the charge movement perpendicular to the magnetic field, the Lorentz forces act leading to particle excursions at the surface of the work piece and thus to the radiation of horizontally polarized transverse waves in the direction of the channels of the comb-like coil support. In a preferred embodiment in which pole assignment of the permanent magnets changes in the adjacent channels, acting-in-the-same-direction Lorentz forces act transverse to the parallel direction of the channels and fins.
In contrast to the transducer of DE 36 37 366 A1, in the present transducer the sonic radiation direction is oriented parallel to the fins respectively to the channels of the comb structure. Furthermore, the present transducer is based on the Lorentz principle, whereas in operation the DE 36 37 366 A1 wave transducer acts as a SH wave transducer via a pure magnetostriction mechanism. Due to the utilization of Lorentz forces in generating ultrasonic waves with the present transducer, ultrasound generation is substantially less dependent on the properties of the material than the SH wave transducer of DE 36 37 366 A1. Furthermore, not only ferromagnetic materials, but also all other electrically conductive materials can be tested with the present transducer.
Of course, the present transducer can be operated not only as a transmitter transducer in which the HF coils are fed HF current from a high-frequency generator. But rather the transducer is also suited to receive ultrasonic waves in reverse direction in the work piece with the same coil system and to pass the respective high-frequency signals on to an evaluation unit via the HF coils. In the same manner, different coil systems for transmitting and receiving ultrasonic waves can also be provided on the same comb-like support. In the latter case, the HF coils for receiving ultrasonic waves preferably have twice the number of coil windings than the HF coils for generating ultrasonic waves.
Further details on generating and receiving ultrasonic waves using such a type of electromagnetic ultrasonic transducer, also referred to as an electrodynamic ultrasonic transducer, are not described because they are familiar to someone skilled in the art from DE 36 37 366 A1 or EP 0 609 754 A2.
In one preferred embodiment of the present ultrasonic transducer, in which the permanent magnets of the rows of permanent magnets of adjacent channels are disposed on the same level, i.e. at the same distance to a line perpendicular to the channels respectively the fins, two-sided sound radiation is realized which can be of use in certain applications. However, by means of a special arrangement of the permanent magnets respectively of rows of permanent magnets, also one-sided sound radiation can be realized by an invented ultrasonic transducer in which the adjacent rows of permanent magnets are disposed shifted half the width of a magnet respectively in relation to a base line running perpendicular to the channel. In addition, two separate HF coil arrangements are employed with which one half of the fins of the coil support is impinged. As in this embodiment the permanent magnets of the adjacent channels are spatially staggered xcex/4 of a transducer periodicity, a phase difference of 180xc2x0 of the signals, i.e. erasure of the signals, in one radiation respectively one reception direction of the transducer and structural overlapping in the other radiation direction are the result. This leads to a one-sided radiation characteristic respectively a one-sided reception characteristic of the transducer.
In another advantageous embodiment of the present transducer, one half of the fins are wound with a coil arrangement, the other half with a coil arrangement which can be separately triggered. The permanent magnets in each channel of a coil arrangement are alternately poled in the direction perpendicular to the channels. The same applies to the permanent magnets of the other coil arrangement, however in the middle of the coil support two adjacent rows of magnets have the same pole assignment in a direction perpendicular to the channels. Both coil arrangements are connected to the inputs of a differential amplifier, with the reception signals of the coils possessing a phase difference of 180xc2x0. In this manner, a reception transducer is realized in differential technology.
The number of fins and channels of the coil support of the present ultrasonic transducer can be selected as desired. However, at least three fins should be provided so that two rows of permanent magnets are disposed parallel to each other. In an embodiment for generating a one-sided directional characteristic, at least five should be provided so that there are four parallel rows of permanent magnets. The material of the coil support should have good magnetic conductivity, i.e. be highly permeable and possess poor electrical conductivity. The width of the fins may correspond about to the width of the rows of permanent magnets. The same applies to the height of the fins. Of course, the height of the permanent magnets can also be less or a bit greater than the height of the fins as long as generation of the ultrasonic waves in the work piece is still ensured by the Lorentz forces. The periodicity of the ultrasonic transducer is determined by the length of the single permanent magnets so that someone skilled in the art determines the length as suited according to the desired transducer periodicity. The aperture width of the transducer is determined by the area over which the fins and the permanent magnets disposed in channels extend and is also selected according to the intended application.
Of course, the geometric shape of the fins and of the channels is not limited to a rectangular cross section. But rather the shape of these fins and channels may deviate from a rectangular cross section without impairing the function of the present transducer. The coil windings wound on the fins are preferably provided with an insulation coat in order to prevent an electric short circuit.